When you accept a funding option, you'll land on the Closing requirements page. Look for the header that says "You've selected a funding option for [Lender Name]". You'll see the word "here" hyperlinked — click on it.
A prompt will then appear asking, "Are you sure you want to accept a different financing option?" Select "Yes" to unselect your current choice. Don't worry, any documents you've already uploaded will remain saved.
You'll then be taken back to the financing options page, where you can choose another published funding option.